1. Field of the Invention:
This invention relates to A.C. electric motors, and in particular, to an arrangement for detecting the direction of rotation of an electric motor.
2. Description of the Prior Art:
In general, direction of rotation of an A.C. electric motor is a parameter which is usually not critical to the operation of the motor itself. For this reason, manufacturing procedures are such that no strict supervision is utilized to guarantee that the direction of rotation of the electric motor be precisely determined. Since the possible sources during construction which affect the rotation of the motor are many, and since the reasons for determining the direction of rotation are not important, the direction of rotation of a fabricated A.C. electric motor is usally unknown.
However, for example, when an electric motor is to be utilized in conjunction with a starting package for a gas turbine power plant, the direction of rotation of the electric motor assumes critical importance. This is due to the type of apparatus usually associated with the motor in the starting package. Associated apparatus, such as an oil pump, or a torque converter, is usually quite sensitive to the direction of rotation of its drive motors. It is possible in some situations to severely damage the associated apparatus by impressing thereon a direction of rotation in opposition to the direction of rotation that is desired.
For this reason, it is imperative that the direction of rotation of the electric motor be ascertained before it is energized and employed in the gas turbine starting package. In the prior art, one method of determining the direction of the electric motor in the starting package was to first de-couple the motor from its associated apparatus. The motor could then be energized and its direction of rotation ascertained. If the direction of rotation was incorrect, the simple expendient of changing the leads from the energy source to the starter motor would insure correct direction of rotation. However, this method proves disadvantageous in that it involves the time-consuming, and therefore costly, de-coupling procedures before the starter motor can be isolated from the associated apparatus tested. This of course, assumes that the coupling is easily reachable by work personnel. Once the proper rotation is insured, the motor must be re-coupled to the associated apparatus, with the attendant time-consumption and costs.
The prior art has also determined the direction of motor rotation by turning the starter motor while it is still connected to the associated apparatus. However, as mentioned earlier, this method of determining direction is not always advantageous in that it is possible to severely damage the associated apparatus by rotating it in a direction not in accord with the desired direction of rotation. Such a hit-and-miss test strategy, with the attendant danger of damage to the associated apparatus, is clearly not an effective method of determining motor rotation.
A third method of determining rotational direction of an electric motor utilized by the prior art is to connect a galvanometer, or another similar device, to the terminals of the electric motor. The motor shaft was then manually rotated and the current so induced is detected by the galvanometer and the direction of motor rotation could be determined by the deflection of the galvanometer needle. This method is useful when working with motors of smaller sizes, but is extremely cumbersome when utilized with the large, 1500 horsepower motors utilized in starting packages for a gas turbine power plants.